Ophiodaphne formata is a conspicuously dimorphic ophiuroid; the disk diameters are approximately 1 mm for males and 5 mm for females. The dwarf male clings to the larger female, with the oral surfaces and bursae of the paired ophiuroids closely appressed. Moreover, the female of each pair adheres aborally to the oral surface of a host sand dollar, Astriclypeus manni. Spawning and external fertilization occur in August, at Tsuruga Bay, Sea of Japan. Development of the dimorphic brittle star O. formata is described for the first time, from spawning through metamorphosis, with special attention to the formation of the skeletal system and the external morphology of early juveniles. Fertilized eggs are about 90 [micro]m in diameter, pale pink, and negatively buoyant. The embryos undergo equal, total, and radial cleavage, and the larval skeleton first forms as a pair of tetraradiate spicules. Larval development proceeds to an 8-armed planktotrophic ophiopluteus, with skeletal elements that consist of a body rod and two recurrent rods. Three weeks after fertilization, all the pluteal arms, except for the postero-lateral arms, are absorbed, and the metamorphosing larvae sink to the bottom. Metamorphosis is completed 21.5 days after fertilization, and the resulting juvenile is pentagonal and approximately 270 [micro]m in diameter. The smallest specimen (480 [micro]m in disk diameter) collected by field sampling exhibited male features on the skeletal plates of the jaw and disk. Sexual dimorphism, the peculiar pairing behavior, and the close relationship with the host sand dollar may have evolved as distinct reproductive characteristics in this ophiuroid with its typical ophiopluteus larvae.